In the case of an electronic part such as a ceramic capacitor constituted by laminating a dielectric sheet, a plastic porous sheet serving as a sheet for suction and fixation for delivery is used as an additional laminated member for suction and fixation for delivery of the dielectric sheet.
As the porous sheet, a porous sheet consisting of ultrahigh-molecular-weight polyethylene having an average molecular weight of 500,000 or more (referred to hereinafter as “UHMWPE”) is proposed to be used in consideration of air permeability, rigidity, and cushioning properties.
Generally, the porous sheet consisting of UHMWPE is produced by charging a mold with UHMWPE and then subjecting it to sintering etc. However, this method constitutes batch production and cannot produce a continuous porous sheet successively.
Accordingly, the applicant has previously proposed a method of producing a continuous porous sheet characterized in that UHMWPE powder filled in a mold is sintered with heated water vapor, then cooled and cut (see, for example, JP-B 5-66855).
The porous sheet obtained by this method is continuous and is thus characterized by being unable in various applications, highly strong, and excellent in air permeability.
The porous sheet produced by this method is about 2.0 μm in surface roughness. This is attributable to cutting conducted in the production process. For example, when a porous sheet is produced using fine particles having an average particle diameter of 30 μm or less, there are problem such as generation of pinholes and formation of cracks during filling and after molding, thus making molding difficult.
As a countermeasure against surface roughness, therefore, there are proposed methods of smoothing a surface by lamination with a plastic film and subsequent heating (see, for example, JP-A 09-174694 and JP-A 2001-28390). These methods can be used to improve surface smoothness. At present, however, there is demand for further improvement of surface smoothness.
As a method of molding small-diameter particles, there is disclosed a method which comprises coating a carrier sheet with a dispersion having plastic particles in a solvent, drying it to form a coating thereon, then fusing contact points of the particles, and releasing the coating from the carrier sheet to give a porous sheet (see, for example, JP-A 2001-172577).
In the method described above, small-diameter particles can be formed into a sheet. The method is applied to cases where the sheet is evidently inferior in strength to a porous sheet produced by cutting. From the viewpoint of the process, the method is more suitable for production of a porous thin sheet than for production of a heavy-gage sheet of greater than 1 mm, for example.